A new study by a team researchers including UTSC Professor Myrna Simpson reveals that as global temperatures rise the organic matter in forests appears to be breaking down more quickly, accelerating the release of carbon into the atmosphere. (Photo by Ken Jones)

Forests can store as much as 45 percent of the world’s
terrestrial carbon, making them a critical part of the process of
regulating climate change. As global temperatures rise, though, the
organic matter in forests appears to break down more quickly,
accelerating the release of carbon into the atmosphere.

This surprising conclusion comes out of a long-term study that was
intended to find means to mitigate global warming, not exacerbate
it.

“Our question was, ‘How much carbon can the soil
hold?’” says UTSC professor of environmental chemistry,
Myrna Simpson. “But in our experiments, we found that soil was
not the limiting factor. We couldn’t even get to the carbon
saturation point.”

Since 1990, a team of international scientists have been running
experiments in Harvard Forest in Massachusetts, testing the effect of
adding (or removing) varying amounts and types of “litter”
– leaves, twigs, seeds, roots and other organic material –
above and below ground. Simpson joined this work in 2010. She
contributed specialized expertise in nuclear magnetic resonance (NMR)
spectroscopy to the mix. NMR allows researchers to scan every type of
organic material in soil, molecule by molecule.

“The scientific community widely accepts that soil organic
matter chemistry is tied to inputs,” she says. “But we
were surprised to see that all of our litter manipulation resulted in
accelerated breakdown of organic matter.”

Climate change could lead to “more productive” forests
– bigger trees and more vegetation. This productivity would
naturally increase the amount of litter, and therefore the amount of
carbon sinking into the soil in the form of organic matter.

But in a paper published recently in the journal
Biogeochemistry, Simpson and her co-authors describe how they
simulated this change by doubling the amount of litter in sections of
the forest in the hope that the soil could absorb more carbon.
Instead, the increased litter stimulated bacterial and fungal
activity. Organic matter broke down more quickly, eliminating any
carbon storage benefit and releasing more CO2 into the
atmosphere.

“Altering the litter did more harm than good,” Simpson
says. “Ours was a human manipulation, but it could as easily be
altered through climate change.”

Simpson’s experiments continue both at the Harvard Forest and
at other experimental forests around the world in collaboration with a
large network of ecologists and soil scientists. In each case, local
plant species, climate and other factors might lead to different
results. Also, litter is just one consideration in how long carbon
stays sunk in a forest – Simpson is testing the effects of
nitrogen and other variables that could affect forests’
abilities to store carbon.

Soil breakdown is further complicated because plants create many
products – from cellulose to lignin – each of which is
affected differently by changes in soil content and environmental
conditions. What accelerates one form of decomposition might slow down
another.

“I want to emphasize that this was just one forest. We
don’t know if this is a global phenomenon,” she says.
“We’re looking now to see how vegetation, temperature,
moisture in different regions affects the process. These results just
suggest that for forests like the Harvard Forest, adding extra litter
is not a way to mitigate climate change and enhance carbon
storage.”

Simpson’s work is supported through the NSERC Discovery Grant
and Discovery Accelerator Supplement programs.

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